Conventional injection molding processes have been modified to create composite structural members that combine polymers with filler materials. For instance, composite shingles have been fabricated in a closed molding process utilizing various combinations of rubber (e.g., ground up tire rubber), thermoplastics (e.g., polyolefins, polyvinyl chloride, etc.) or other polymers, and fillers (e.g., glass, stone, limestone, talc, mica, cellulosic materials such as wood flour, rice hulls, etc.), along with colorants, and optionally, suitable UV inhibitors, lubricants and other additives that aid in the molding process and provide favorable physical properties to the finished composite shingles (e.g., heat reflection, certain weathering characteristics, physical strength, etc.). Two popular types of composite shingles formed in a closed molding process including composite shake shingles and composite slate shingles. Different mold tools are created for each type of composite shingle, each with its own surface texturing or contouring to be imparted to the molded article.
One general formulation that has been found to be suitable for a molded composite shingle is to use around 35-70% polymer and 30-65% filler by weight of the components, along with small amounts of colorants and other additives. More specifically, polymers that have been found to be useful in forming composite shingles include polyethylene or other polyolefins, or polyvinyl chloride, as well as crushed limestone or a similar stone as a filler. Using such a high percentage of stone or other relatively abrasive fillers in a material feed within a closed molding process can be problematic however. With conventional injection molding, a highly flowable, generally low abrasive polymer resin is heated and moved through various ports into a molding cavity under pressure. To get good flow characteristics from a material feed containing a high amount of abrasive fillers, the feed must also be heated and maintained at an elevated temperature until it fills the mold cavity where an article is being formed. Furthermore, the temperature at which the material feed with abrasive fillers must be maintained to achieve good flow may not be the same as the ideal curing temperature for the article in the mold cavity. Thus, it can be difficult to achieve desired heating characteristics when certain flow paths are utilized in molding these types of composite articles.
One prior surface mold tool construction used to form composite shingles is illustrated in FIG. 1. The tool 10 depicted is only the “A” member of a molding device, in this case the female member having a base surface 12 where one or more concave article molding regions 14 are disposed to form a top surface and side edges of a molded composite slate shingle. An opposed “B” tool member is not shown, but is generally designed to mirror the perimeter edge 16 of the “A” tool 10 at the base surface 12 to enclose the article molding region 14 for curing of the molding material feed into the finished shingle article. The “B” tool member has its own article molding regions to establish the shape of a bottom surface of the composite slate shingle, and may be flat, recessed below or extending above a base surface thereof. To deliver the material feed to the article molding regions 14 of the “A” member tool 10 and the “B” member tool, combining to form molding cavities (not shown), a flow channel 18 is formed into the base surface 12 of at least the “A” member tool 10. The flow of material feed to a start point 20 of the flow channel 18 may be accomplished by various distribution channels (not shown), either internal or external to the “A” and “B” member tools.
With the design of the “A” member tool 10 of the prior art, the abrasive material feed flow must travel from the start point 20 up a vertical section 22 of the flow channel 18, and then turn down transverse sections 24 to enter one of two article molding regions 14. This type of flow pattern is problematic in that the abrasive material of the material feed wears heavily on surfaces of the “A” and “B” member tools where the flow is caused to change direction. The flow channel 18 also makes it difficult to maintain the material feed at the optimal temperature for flowing into the molding cavity (i.e., the article molding region 14). Another problem is that the conventional flow channel design creates an external “gate” along the sides of the molded composite article. When the molded composite articles are curing within the molding cavity, the material feed present in the flow channel also cures to form the gate. This extra piece interconnects adjacent molded composite shingles and must be later removed because it serves no useful purpose as a roofing product. A robotic mechanism that removes the cured shingles from the article molding regions must also take the shingles to a location where the gate is broken off to reveal the finished roofing product. This process can require complex robotic movements if it is desired to move the shingles to a location where they can be packaged without having to put down the shingles to remove the gate, and then pick them back up again. Furthermore, it is often difficult to recycle the waste gate pieces for use in another molding cycle. This is because the abrasive material makeup (e.g., stone) causes extensive wear on regrinding or other processing equipment that conditions the gates for reuse as raw material feed.